Using data from GovTrack, we looked at every vote taken in the House and Senate so far in the 114th Congress. We figured out the majority position for each party (in cases where it was not unanimous) and compared every member of each body's vote against the party majority.
The member of the House most willing to buck his party is Walter Jones (R-N.C.), who has voted against the Republican majority more than one-third of the time. On the Democratic side, the high-water mark is Collin Peterson (D-Minn.), who's at 30 percent.
In the Senate, Joe Manchin (D-W.V.) voted against the rest of the Democrats more than 25 percent of the time. But Susan Collins (R-Maine) was even more of a maverick, at nearly 27 percent. (The original maverick, John McCain, is only at 8 percent so far this Congress. His protege, Lindsey O. Graham, cracked the top 10 for Republicans.)
Update: We should have mentioned why Mitch McConnell is on this list! The leader of the Republican Party, voting against his party? As we've noted before, a motion to reconsider a cloture vote can only be filed if the person has "changed his mind" on the vote. So McConnell votes against the party so that he can later "change his mind" and bring the topic back up. It's procedure, not sincerity.
The image above considers all votes, including ones where going against the party doesn't really matter that much. If you look only at Senate votes where the majority got less than 66 votes or House votes where the winners were under 235, the scene changes a bit.
For example, Manchin was more likely to vote against the Democrats in closer votes (although on the Senate side, the number of non-close votes was pretty small). That held for the rest of the Democrats, too.
On the House side, where being in the minority offers less power, the Democrats that voted the most against the party were less likely to do so when the vote was close. But Republicans like Christopher P. Gibson and Bob Dold were much more likely to.
There have been seven House votes so far this Congress in which the winning side had fewer than 218 votes. Dold voted against his party six of the seven times. Gibson voted against the Republicans every single time.
You will not be surprised to learn that their districts have been rated toss-ups by Cook Political Report. Or that Gibson is retiring.